The use of a direct coupling of high efficiency liquid chromatography and gas chromatography is very useful for analyzing complex mixtures. The advantages in using this multidimensional system basically centre on the possibility of combining the liquid chromatography potential as a sample preparation technique with that of gas chromatography in relation to the system""s efficiency (Grob. K., On-Line Coupled LC-GC., Hxc3xcthig, Heidelberg, Germany, 1991; Mondello, L.,; Dugo, G.; Bartle, K. D., J. Microcol, Sep., 1996, 8, 275-310). In this way, it is possible to avail of analysis methods which do not require the use of conventional sample preparation procedures which, apart from being laborious and unreliable, have the great disadvantage of calling for the use of relatively high volumes of polluting organic solvents.
A particularly problematic aspect in connection with the use of direct coupling of liquid chromatography and gas chromatography relates to the characteristics of the interface necessary to make this coupling possible. This is an aspect which displays the difficulty in making two essentially different systems, where the operating parameters are substantially different, compatible. The interfaces initially developed only allowed the use of a normal phase in the pre-separation performed by liquids since, in this case, the small volumes of vaporization produced during the transfer do not lead to any additional difficulties. This is why different interfaces (xe2x80x9cautosamplerxe2x80x9d, xe2x80x9con-columnxe2x80x9d, xe2x80x9cloop typexe2x80x9d) enabling direct coupling to be carried out between liquid chromatography in the normal phase and gas chromatography (Grob, K. J. Chromatogr. A 1995, 703, 265-76; Vreuls, J. J.,; de Jong, G. J.; Ghijsen, R. T.; Brinkman, U. A. Th. J. AOAC. Int 1994, 77, 306-27) have been designed and used.
However, it is necessary in many cases to turn up to the use of the reverse phase in the liquid chromatography stage in order to achieve a certain separation and, consequently, the extension of the field of applicability of direct liquid chromatography and gas chromatography coupling requires the development of suitable interfaces for carrying out direct coupling between liquid chromatography in a reverse phase and gas chromatography (Se{overscore (n)}orxc3xa1ns, F. J.,; Villxc3xa9n, J; Tabera, J.; Herraiz, M. J. Agric. Food Chem. 1998, 46, 1022-27. Villxc3xa9n, J; Blanch, G. P.; Ruiz del Castillo, M. L.; Herraiz, M. J. Agric. Food Chem. 1998, 46, 1027-31. With this aim in mind, several systems have been proposed over the last few years (xe2x80x9cretention gapxe2x80x9d, xe2x80x9cconcurrent solvent evaporationxe2x80x9d, xe2x80x9copen tubular trapxe2x80x9d, etc.) (Grob, K. J. Chromatogr. A 1995, 703, 265-76; Vreuls, J. J.; de Jong, G. J.; Ghijsen, R. T.; Brinkman, U. A. Th., J. AOAC. Int. 1994, 77, 306-27) although the limitations involved in using polar eluents (fundamentally the high volumes of vaporization produced during transfer and the difficulty of suitably focussing the chromatographic band) have prevented the development of interfaces meeting the required conditions as regards simplicity, reliability, versatility and possibility of automation.
Interfaces described up to date in literature have limitations in some of the following aspects: The liquid chromatography fraction""s volume that can be transferred, the flow rate at which this fraction can be transferred, the impossibility of making the transfer when liquid chromatography is performed in the reverse phase, or the system cannot be automated.
The liquid chromatography fraction volume normally transferred with the systems described is about 500 microliters and rarely exceeds one milliliter and then only by a little. This makes it necessary to transfer only a part of the liquid chromatography fraction of interest or leads to working with very small diameter columns in liquid chromatography (liquid microchromatography), which means experimental difficulties and a loss of sensitivity. Some interface allowing much greater volumes to be transferred has been described, but it cannot be automated. In the device which is the subject of the invention, the volume of the fraction transferred is unlimited. Up to 100 ml have been inserted, a much higher volume than any liquid chromatography fraction it is desired to transfer with analytical ends, but this volume can be far exceeded when required.
The flow rate at which the transfer is performed is limited by the solvent evaporation rate in most of the systems described. This also makes it necessary to work in liquid microchromatography or to use an interface having a system which collects the liquid chromatography fraction and transfers it to gas chromatography at a lower flow rate at which it elutes from liquid chromatography, and this increases the complexity of the interface. Some interface has been described which allows transfer to be made at a much greater flow rate than the rest, but it cannot be automated. In the device which is the subject of the invention, the flow rate at which the liquid chromatography fraction is transferred is far higher than any of the systems described which may be automated. Transfers have been made at 5 milliliters per minute, a far higher rate than that necessary to transfer a liquid chromatography fraction with analytical ends, but this rate may be far exceeded if required.
Most of the interfaces described only allow the transfer to be made when liquid chromatography is performed in a normal phase. This means a limitation in itself, which is substantial if it is borne in mind that the immense majority of liquid chromatography separations have been carried out in the reverse phase. The interfaces described which allow liquid chromatography fractions to be transferred in the reverse phase make a change in solvent during the transfer or cannot be automated or have the limitations as discussed earlier. In the device which is the subject of this invention, liquid chromatography can be performed both in the normal and in reverse phases. It also allows very high volumes of aqueous solution to be transferred at a high flow rate.
The difficulty in achieving direct coupling of liquid chromatography and gas chromatography lies in the fact that the maximum volume of sample that can be inserted into a gas chromatography capillary column is in the order of one microliter, whilst a liquid chromatography fraction of interest normally has a volume between about 100 microliters and a few milliliters. Therefore, the interface employed for this direct coupling has to evaporate the solvent to the volume admissible in gas chromatography, retain the solutes of interest and transfer them to the gas chromatography column occupying a narrow band of the column, so that chromatographic separation is effective.
An increase of analysis sensitivity is achieved by introducing high volumes of sample or extract in gas chromatography. Due to this increase in sensitivity, the extraction and concentration processes prior to the gas chromatography analysis may be replaced in many cases by introducing high volumes of sample. The difficulty with this technique is basically the same as that of liquid chromatography and gas chromatography coupling, i.e. to evaporate the solvent from the high volume inserted until the solution has a volume lower than the maximum admissible by the gas chromatography capillary column, retain the solutes and move them into the gas chromatography column.
Therefore, if there is no specific instrumental difficulty, any device suited for using as an interface for direct liquid chromatography and gas chromatography coupling is also suited to the introduction of high volumes of sample in gas chromatography, and vice-versa.
The device according to the present invention is built on the basis of the scheme of a PTV (programmed temperature vaporizer) injector in which the system for inserting the sample, the hydraulic gas system and the operating mode have been modified. Thus, the present invention provides an interface device for direct coupling of liquid chromatography and gas chromatography based on a programmed temperature vaporizer (PTV), the device being capable of operating at least in an adsorption mode and in a desorption mode, and comprising
an injector divided into a first inner part and a second inner part and which houses a glass liner containing an adsorbent, the glass liner having a first end portion within the first inner part and a second end portion within the second inner part of the injector;
a system for selecting a liquid chromatography fraction and conducting the liquid chromatography fraction to the glass liner, said system comprising a first tube protruding into the glass liner and a first valve connected to a second end of the first tube;
a discharge system for discharging the liquid chromatography fraction into the glass liner and for preventing, when the device operates in the adsorption mode, the liquid chromatography fraction from entering a gas chromatography column, the discharge system comprising
a first end of the first tube terminating within the glass liner at a first distance from the adsorbent, the gas chromatography column having an inlet protruding into the glass liner by said first end portion, said inlet being located within the glass liner at a second distance from the adsorbent, said first distance being shorter than said second distance,
a first gas inlet for entry of a pressurized gas flow into said first inner part of the injector,
a second gas inlet for entry of pressurized gas into said second inner part of the injector, and
a second tube protruding into the glass liner by said second end portion;
an outlet system for evacuating solvent from the glass liner, the outlet system comprising an outlet valve connected within the second tube, the outlet valve being closed in the desorption mode and open in the adsorption mode; and
a modified hydraulic system for gases connected to the first gas inlet and to the second gas inlet and to a gas tank containing pressurized gas for providing the pressurized gas flow.
The operation of the device during the transfer of the liquid chromatography fraction of interest, is based on two principles: adsorption in the solid phase and solvent evaporation. The thermal desorption of the solutes retained takes place subsequently.
The glass liner of the PTV injector is filled with an adsorbent. The inlet end of the capillary chromatographic column is inserted into the injector in the usual way. The first end of the first tube is also inserted at the same place up to the glass liner. It will carry the fraction from liquid chromatography down to a depth deeper than the inlet end of the gas chromatography column. This first tube is connected at the end opposite to the liquid chromatograph through a system of valves allowing the liquid chromatography fraction of interest to be selected, and the eluent remaining in the transfer tube or capillary when the transfer has finished to be removed.
The second tube protrudes outside the injector and takes the solvent removed to a waste, and can have an opening and closing valve and an intermediate opening and closing valve, and it is inserted through the opposite end of the glass liner.
The hydraulic gas chromatograph system is modified by a system of pressure reducers, opening and closing valves and flow controlling valves so that it is allowed to operate in two modes, adsorption or desorption.
Controlled flows of gas may be sent in through both ends of the glass liner in the adsorption mode. This mode is used during the transfer and subsequent disposal of solvent remains. The gas entering the glass liner through the same place as the first tube pushes the liquid towards the adsorbent and draws the solvent along to the second tube. The gas entering the glass liner through the opposite end prevents the injector from being flooded.
In the desorption mode, gas only reaches the glass liner through the inlet used in the usual configurations, at a controlled pressure. The thermally desorbed solutes are taken by this flow of gas to the capillary column where the chromatographic process takes place.
As readily apparent, contrary to prior art interfaces, the devices which is the objet of the present invention allows a very large volume liquid chromatography fraction to be transferred (or directly injected) at a very high flow rate, allows liquid chromatography fractions in a normal phase or reverse phase to be transferred and can be automated. It also allows large volumes of sample or extract to be directly inserted, both with polar and non-polar solvents.